When someone who you like and respect does something bad that surprises you, you use this phrase. For example, if your husband yells and says something mean to your sister, you would tell him:

I can't believe you would say that to her!

In that situation, you can also say:

I can't believe you said that!

But there is a small difference between the two. "I can't believe you would ___" sounds more surprised and hurt.

You can also use "I can't believe you (did something)" for a surprising positive action. For example, if your wife bought you a really amazing and expensive birthday gift, you could say:

I can't believe you got me a motorcycle!

If you said "I can't believe you would get me a motorcycle", it would definitely sound like you were angry at her.